Trilayer, shutdown battery separators are known. See: Japanese Patent Application Nos. 98395/1994 and 98394/1994 both filed May 20, 1994 by Ube Industries, Ltd. (the Ube trilayer separator); U.S. patent application Ser. No. 359,772 filed Dec. 20, 1994, now abandoned, by Hoechst Celanese Corporation (the CELGARD.RTM. Separator); Japanese Patent Application No. 55550/1995 filed Mar. 15, 1995 by Nitto Denko Kogyo K.K. (the Nitto Denko trilayer separator); and Japanese Patent Application No. 56320/1995 filed Mar. 15, 1995 by Kureha Chemical Industry Co., Ltd. (the Kureha trilayer separator).
Trilayer, shutdown battery separator are porous films and have become a commercial success, within the battery industry, particularly within the secondary (or rechargeable) lithium battery industry. The success is believed to be due, in part, to the separator's puncture strength and the separator's ability to reduce the risk of internal short circuit within the battery. The separator's puncture strength is, at least, important because it enables the separator to withstand the rigors of battery manufacture. The separator's ability to reduce the risk of internal short circuit is believed to be important because internal short circuit may lead to unexpected battery rupture.
The Ube trilayer separator is a porous trilayer film having a polypropylene-polyethylene-polypropylene construction. Ube's process includes the steps of: extruding a polypropylene non-porous precursor; extruding a polyethylene non-porous precursor; forming the polypropylene-polyethylene-polypropylene non-porous trilayer precursor; bonding the trilayer precursor together at a temperature ranging from 120-140.degree. C. between nip rollers and then taking up the precursor for subsequent processing; heat-treating (annealing) the bonded precursor at a temperature range from 110-140.degree. C.; and stretching the bonded, annealed precursor to form the porous, trilayer separator. The stretching step consisted of three discrete steps: cold-stretching (temperature ranging from -20.degree. to 50.degree. C. and a draw ratio of 5-200%); hot-stretching (temperature ranging from 70-130.degree. C. and a draw ratio of 100-400%); and heat-treating, also believed to be referred to as annealing and/or relax (temperature ranging from 75-175.degree. C. and a draft ratio of -10 to -50% or 0%).
The CELGARD.RTM. separator is a porous trilayer film having a polypropylene-polyethylene-polypropylene construction. The CELGARD separator process includes the steps of: forming a porous polypropylene precursor; forming a porous polyethylene precursor; forming a porous trilayer precursor; and bonding the porous trilayer precursor to form the trilayer battery separator.
The Nitto Denko separator is a porous trilayer film having polypropylene-polyethylene-polypropylene construction. The Nitto Denko separator process includes the steps of: co-extruding a trilayer precursor having a polypropylene-polyethylene-polypropylene non-porous construction; annealing (heat-treating) the trilayer precursor; and stretching the annealed trilayer precursor to form the porous trilayer battery separator.
The Kureha separator is a porous trilayer film having a polypropylene-polyethylene-polypropylene construction. The Kureha separator process includes the steps of: co-extruding a trilayer precursor, the precursor containing a pore forming aid (e.g., a fine inorganic particle or a solvent extractable material); and forming the pores in the precursor by stretching the particle-loaded precursor or by solvent extraction of the extractable-laden precursor.
There is a need to continue to improve the process for making trilayer shutdown separators so that: manufacturing throughput can be increased while minimizing the capital investment; and manufacturing cost can be minimized by minimizing material handling costs and increasing production quality.